Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also useful for cracking hydrocarbons and/or executing thermal processes, including some processes conducted at temperatures higher than can suitably be performed in conventional steam crackers. Compared to conventional cracking equipment and processes, higher temperature reactions (e.g., >1500° C.) and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Economical operation of high severity hydrocarbon cracking processes and equipment requires satisfying numerous competing operational and engineering challenges. Properties such as melt temperature, reaction environment non-inertness, component strength, and toughness limitations commonly define limits for many processes. The high temperatures and process stresses can exceed the long term viability of most conventional apparatus, including conventional refractory ceramics. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
In high thermal stress applications, many ceramic materials are subject to progressive failure due to onset mechanical and chemical changes within the ceramic matrix. For example, thermal cycles and related stress fluctuations may induce progressive formation of micro-fractures that may continue to grow and disperse until reaching a critical threshold resulting in premature component failure. In addition to physical and thermal performance considerations, component chemical inertness and crystalline stability also are significant considerations. Many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, in the presence of certain elements such as carbon and oxygen. The increased chemical activity at increased temperature may lead to premature degradation and/or process interference such as by generation of unacceptable levels of contaminants in the process. The presence of carbon from the hydrocarbon feedstock and potential presence of oxygen from within the reaction process, at severe pyrolysis temperatures, present special ceramic-metallurgical crystalline-stability challenges to avoid premature ceramic corrosion. This corrosion can be detrimental in that it may favor initiation and/or sustaining micro-fracture growth and dispersion.
Prior art regenerative pyrolysis reactor systems commonly used alumina as the bed packing material. The commercial embodiments of these reactor systems did not operate at temperatures sufficient to achieve high conversion of methane feed. One reason for this is that pure alumina has a melting point of 2050° C. and practical alumina may have lower melting point due to impurities. Further, maximum practical use temperatures are typically two- to three-hundred degrees lower than the actual melting temperature, which combined with decreases due to impurities, renders alumina unsuitable for many uses in a high temperature (e.g., >1500° C., or >1600° C., or up to 2000° C.) pyrolysis reactor. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors.
A similar, recently recognized problem particular to high temperature hydrocarbon pyrolysis pertains to carburization within the ceramic component, which can produce carbide-oxide conversion chemistry in many ceramic materials, such as some zirconia oxides, that also leads to progressive component degradation, herein considered a type of “ceramic corrosion.” This high temperature hydrocarbon-related corrosion mechanism was not previously identified, understood, or recognized as a concern with high temperature hydrocarbon pyrolysis using ceramics. Carburization is a heat activated process in which a material, such as a ceramic or metal, is heated to temperature below its melting point, in the presence of another material that liberates carbon as it thermally decomposes, such as hydrocarbons. The liberated carbon can permeate the exposed surface and near-surface interior of the ceramic crystal matrix and either remains in spatial regions as coke or at more elevated temperatures react with the ceramic to form ceramic carbides. Such permeation by carbon can over time adversely affect the mechanical and chemical properties of the ceramic material such as are otherwise needed for long-term use in commercial, hydrocarbon pyrolysis reactors. Ceramic component volatility and progressive loss due to the severe temperatures and cyclic temperature swings also may contribute to carburization. Issues include carbon infiltration and coking within the ceramic matrix pores and an associated, undesirable carbide-oxide interaction chemistry resulting in progressive corrosion and degradation of the ceramic matrix, including micro-fractures due to coke expansion. Such problems are of particular interest in high severity pyrolysis of hydrocarbon feedstocks (e.g., >1500° C.).
The pyrolysis art needs a ceramic composition that resists or avoids carbon permeation, carburization, and/or oxide-carbide corrosion, while also providing a mechanism to arrest micro-fracture propagation should such fractures be initiated. The desired materials should concurrently provide and maintain the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness required for large scale, commercial applications, particularly with respect to hydrocarbon pyrolysis.